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Urinary tract infection (lower) - women - Management
What drug interactions should I be aware of with trimethoprim?
The following drug interactions have been reported with trimethoprim, when it is combined:
- With methotrexate (a folate antagonist):
- Several cases of bone marrow suppression have been reported (some fatal) [Baxter, 2008].
- With azathioprine:
- Increased risk of haematological toxicity has been reported in some people with renal transplant and taking azathioprine — particularly if both drugs are given over a prolonged period [Baxter, 2008].
- Nevertheless, for most people, both drugs can be taken together. The combination is commonly used in practice.
- The reaction is also expected for mercaptopurine (a metabolite of azathioprine).
- With phenytoin:
- There is a small risk of phenytoin toxicity (particularly if the serum phenytoin levels are at the top end of the range) as trimethoprim can decrease the clearance of phenytoin [Baxter, 2008]. Signs of phenytoin toxicity include blurred vision, nystagmus, ataxia, or drowsiness.
- This interaction is also expected with fosphenytoin (a pro-drug of phenytoin).
- With ciclosporin:
- Increased nephrotoxicity has been reported. However, this interaction has not been firmly established [Baxter, 2008].
- With digoxin and warfarin. However the clinical significance of these interactions is still uncertain [Baxter, 2008].
- Digoxin: trimethoprim has been reported to increase digoxin levels by an average of 22% in nine elderly people after taking trimethoprim 200 mg daily for 14 days (although an increase of 75% was experienced by one person). Consider monitoring for signs of digoxin toxicity in the elderly if trimethoprim is given long term for prophylaxis [Baxter, 2008].
- Warfarin: the manufacturer of trimethoprim warns that it may potentiate the anticoagulant effect of warfarin. However, no case reports or controlled trials on this interaction have been reported. Although there is some indication of increased anticoagulant effect from two cohort studies, the interaction is likely to be small (if it occurs), requiring little or no adjustment in warfarin dose [Baxter, 2008].
[Baxter, 2008; BNF 57, 2009]
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